Method of determining the capacity of a cosmetic to diffuse and/or absorb light

ABSTRACT

Methods and apparatus for determining the optical characteristics of cosmetic substances are disclosed. The method includes illuminating the surface of the substance with an incident light beam so as to form a back-scatter spot, obtaining an image of the back-scatter spot, and analyzing the variations in brightness of the image at a number of points so as to determine the optical characteristic in question.

[0001] The present invention relates to a method of determiningmagnitudes characteristic of the optical behavior of a surface on whicha cosmetic has been applied, in particular of the skin and/or ofkeratinous fibers, and to apparatus for implementing the method.

BACKGROUND OF THE INVENTION

[0002] The term cosmetic is used to cover any substance as defined inEEC Directive 76/768 as amended by Directive 93/35 of Jun. 14, 1993.

[0003] The article “Integrating the digitized back-scattered image tomeasure absorption of reduced-scattering coefficients in vivo” by Gobinet al., published in the journal Applied Optics, of Jul. 1, 1999,describes measurements of diffusion and absorption coefficientsperformed in the laboratory on bare skin.

OBJECTS AND SUMMARY OF THE INVENTION

[0004] In one of its aspects, the invention provides a method ofdetermining the capacity for diffusing and/or absorbing light of acosmetic or dermatological product optionally applied on a support, themethod comprising the following steps:

[0005] a) illuminating the substance and/or a zone of the support onwhich the substance has been applied with an incident light beam in sucha manner as to form a back-scatter spot;

[0006] b) acquiring an image of the spot; and

[0007] c) analyzing the image as a function of variation in brightnessbetween the various points of the spot to determine at least one item ofinformation representative of the capacity for diffusing light and/orabsorbing it of the substance and/or the support in the presence of thesubstance.

[0008] By way of example, the substance illuminated by the incidentlight beam may form a relatively thick mass, e.g. being contained in areceptacle. It can also form a layer on the surface of a support, wheresuch a layer can be relatively thin. The substance may also be absorbedby a support.

[0009] In an implementation of the invention, it is thus possible as afunction of variation in the brightness of various points of the spot,and in particular of points situated at different distances from thecenter of the spot, to determine a reduced diffusion coefficient μ′_(s).In a variant, or additionally, it is also possible to determine anabsorption coefficient μ_(a).

[0010] The support may comprise keratinous cells and/or fibers. Thesupport may be inert or living.

[0011] The surface illuminated by the incident light beam may besubstantially plane or it may present relief.

[0012] The substance may have been applied on the support to cover anarea, for example an area lying in the range 0.5 square centimeters(cm²) to 5 cm², and in particular in the range 1 cm² to 2 cm².

[0013] The incident light beam may be a beam of white light, or in avariant, it may be a beam of monochromatic light, in particular visiblelight, e.g. of red or blue color. Its wavelength may be selected as afunction of the nature of the support and/or of the optical propertiesthat are to be determined, for example. The light beam may be a beam ofcoherent light.

[0014] The section of the light beam may be less than or equal to 4micrometers (μm), and preferably less than 2 μm.

[0015] The intensity of the incident light beam is preferablysubstantially constant over its entire section. Since the incident lightbeam is produced by a light source, a spatial filter may be placedbetween the source and the substance.

[0016] The angle of incidence of the incident light beam relative to thenormal of the surface of the substance may lie in the range 5° to 25°,for example, and in particular in the range 10° to 20°.

[0017] Where appropriate, a mirror may be placed on the path of theincident light beam.

[0018] Advantageously, a device without a mirror can be used for in vivomeasurements.

[0019] The image may be acquired in various ways, for example by meansof a camera, in particular a monochrome charge-coupled device (CCD)camera.

[0020] Data processing may be performed locally, but in a variant it ispossible to transmit the acquired image remotely, in particular over theInternet, to a processing center for analysis purposes.

[0021] The image may be acquired through an optical system havingvariable magnification. This makes it possible to enlarge the image sothat it occupies a majority of the sensing area so as to benefit frombest resolution.

[0022] A polarizer may be placed on the path of the incident light beam,between the source and the substance, and the image may be acquiredthrough an analyzer placed between the substance and the camera.

[0023] The spot may be observed in the absence of any interfering light,however the incident light beam may be modulated and the image may beacquired synchronously, thus making it possible to take measurements inthe presence of interfering light.

[0024] The substance may be applied to the support so as to havethickness, in particular substantially constant thickness, lying in therange 7 μm to 20 μm for example.

[0025] The substance may be of uniform composition.

[0026] By way of example, the substance may be selected from thefollowing: foundation makeup, sunscreen, depigmenting cream, antiwrinklecream, moisturizer, this list not being limiting.

[0027] The substance may be suitable for modifying at least onephysico-chemical property of the support on which it is applied, as isthe case for example of a depigmenting cream.

[0028] In another of its aspects, the invention also provides a methodof determining a characteristic of a cosmetic substance, the methodcomprising the steps of:

[0029] determining the capacities of two surfaces to diffuse and/orabsorb light, the surfaces being constituted respectively:

[0030] a) by a substance applied to the skin and/or the keratinousfibers; and

[0031] b) by the skin and/or keratinous fibers in the absence of thesubstance, by performing for each surface at least the following steps:

[0032] i) illuminating a zone of the surface in question with anincident light beam in such a manner as to form a back-scatter spot;

[0033] ii) acquiring an image of the spot; and

[0034] iii) analyzing the image as a function of variation in brightnessat various points of the spot in order to determine at least one item ofinformation representative of the capacity of the substance, the skin,and/or the keratinous fibers to diffuse light and/or to absorb it; and:

[0035] determining at least one characteristic of the substance as afunction of the information obtained for the skin and/or the keratinousfibers with and without the substance.

[0036] The characteristic of the substance as determined in this way maybe its covering ability or its staying power.

[0037] When the substance is a sunscreen, the characteristic which isdetermined may be the effectiveness of the protection conferred thereby.

[0038] With a depigmenting cream, the characteristic which is determinedmay be the effectiveness thereof.

[0039] In another of its aspects, the invention also provides a methodof determining how a characteristic of a cosmetic substance varies overtime, the method comprising the following steps:

[0040] a) applying the substance on a support;

[0041] b) illuminating a zone of the support on which the substance hasbeen applied with an incident light beam in such a manner as to form aback-scatter spot;

[0042] c) acquiring a first image of the spot;

[0043] d) analyzing the first image as a function of variation inbrightness at various points of the spot to determine at least one itemof information representative of the capacity of the substance todiffuse light and/or to absorb it at a first instant;

[0044] e) acquiring a second image of the spot at a second instant;

[0045] f) analyzing the second image as a function of variation in thebrightness at various points of the spot to determine at least one itemof information representative of the capacity of the substance todiffuse light and/or to absorb it, at said second instant; and

[0046] g) determining how a characteristic of the substance varies overtime as a function of the information obtained during steps d) and f).

[0047] In an implementation of the invention, the method may include thefollowing step between steps d) and e):

[0048] exerting some action on the substance and/or the support.

[0049] By way of example, this action may be washing the support. Thismay make it possible, for example, to determine the ability of sunscreenor makeup to withstand water. Some other action may be exerted, forexample exposure to the wind or to ultraviolet radiation or contact withan element onto which the makeup is to be transferred.

[0050] In another of its aspects, the invention provides a method ofprescribing or preparing a cosmetic substance, the method comprising thefollowing steps:

[0051] a) illuminating a zone of the skin with an incident light beam soas to form a back-scatter spot;

[0052] b) acquiring an image of the spot;

[0053] c) analyzing the image as a function of variation in brightnessat various points of the spot to determine at least one item ofinformation representative of the capacity of the skin to diffuse lightand/or to absorb it;

[0054] d) prescribing, at least as a function of said item ofinformation, a cosmetic substance, in particular a substance having thecapacity to diffuse light and/or to absorb light close to the capacitypreviously determined for the skin.

[0055] The cosmetic substance as prepared or prescribed in this way maybe intended, for example, for masking imperfections of the skin and forachieving natural makeup.

[0056] The invention also provides apparatus for determining thecapacity of a cosmetic substance for diffusing and/or absorbing light,which apparatus may comprise:

[0057] a source suitable for delivering a light beam capable of forminga light spot on the substance or on a support on which a substance hasbeen applied;

[0058] a camera, in particular a monochrome CCD camera, enabling atleast one image of the spot to be acquired; and

[0059] an analyzer apparatus for analyzing said image as a function ofvariation in brightness at various points of the spot, in particular atpoints situated at different distances from the center of the spot, todetermine at least one item of information representative of thecapacity of the substance to diffuse light and/or to absorb it.

[0060] The light source may comprise a laser.

[0061] The apparatus may include a spatial filter through which thelight beam passes, and where appropriate a mirror placed on the path ofthe light beam.

[0062] The light source may be capable of emitting at at least twodifferent wavelengths, optionally simultaneously.

[0063] The analysis apparatus may be configured to receive the imageacquired by the camera via a network, in particular the Internet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The invention can be better understood on reading the followingdetailed description of non-limiting implementations and on examiningthe accompanying drawings, in which:

[0065]FIG. 1 is a simplified diagrammatic view of apparatus enabling anexample of the method of the invention to be implemented;

[0066]FIG. 2 shows an example of the image of a back-scatter spot on theskin after a cosmetic substance has been applied;

[0067]FIG. 3 shows an example of the image of a back-scatter spot on theskin before a cosmetic substance has been applied;

[0068]FIG. 4 is a graph for calculating reduced diffusion and absorptioncoefficients;

[0069]FIG. 5 is an example of an image that results from taking thedifference between images obtained in incident light that ishorizontally polarized and back-scattered light that is polarizedhorizontally or vertically; and

[0070]FIG. 6 is a simplified diagrammatic view of other apparatusenabling an example of the method of the invention to be implemented.

MORE DETAILED DESCRIPTION

[0071] In one of its aspects, the invention serves to determine at leastone reduced diffusion coefficient μ′_(s) and/or at least one lightabsorption coefficient μ_(a) by observing a surface on which a cosmeticsubstance has been applied.

[0072] The reduced diffusion coefficient μ′_(s) represents the change inthe spatial distribution of a beam deflected in multiple directions by asurface or by a medium, without change to the frequencies of themonochromatic rays making it up.

[0073] The absorption coefficient μ_(a) represents the decrease in theintensity of the beam on going through a material, its radiant energyhaving been transformed into some other form of energy.

[0074]FIG. 1 shows apparatus 10 for determining a value that isrepresentative of the capacity of a cosmetic substance P for diffusinglight and/or for absorbing light. The substance cosmetic P is applied toa support S, which may be constituted for example by an inert support,by human skin, or keratinous fibers, said list not being limiting.

[0075] In the example shown, the substance P is applied on the support Sat a thickness lying in the range 10 μm to 15 μm.

[0076] A zone of the support S covered in the substance P is illuminatedwith an incident light beam 20 in order to form a back-scatter spot.

[0077] In the example described, the light beam 20 is monochromatic,being delivered by a laser 31.

[0078] In the example described, the wavelength produced by the laser isabout 635 nanometers (nm), however it would not go beyond the ambit ofthe present invention for the incident light beam to be produced by someother type of source and to present a different wavelength, for examplein the blue or the UV regions of the spectrum. By way of example, it isalso possible to use as the light source a helium neon (HeNe) laser, oneor more light-emitting diodes (LEDs), or a source of white light.

[0079] The light beam 20 passes through a spatial filter 32 serving toobtain uniform distribution of the light intensity of the beam.

[0080] In the example described, the support S onto which the substanceP has been applied is placed horizontally and the laser 31 slopes at asmall angle relative to the horizontal. To illuminate the support S witha light beam that slopes at a small angle relative to the normal, amirror 35 is placed on the path of the incident beam 20, said mirrorbeing inclined at about 45° relative to the vertical. The orientation ofthe incident light beam may possibly be modified by varying theinclination of the mirror 33 relative to the structure 34.

[0081] A camera 40 is used to acquire a digital image of the spot formedby the light beam after being reflected in the mirror 33, and an exampleof such an image is shown in FIG. 2. In this example, the cosmetic is afoundation makeup.

[0082]FIG. 3 shows an example of a digital image of the spot formed onthe skin prior to applying the cosmetic.

[0083] In the example described, the camera is a monochromatic CCDcamera having resolution of 490 by 660 pixels, but it would not gobeyond the ambit of the present invention to use a camera havingdifferent resolution, and possibly a color camera.

[0084] The camera is fixed on a stand 41 that is movable along a guide42 parallel to the support S, thus making it possible to adjust thedistance between its lens and the support S.

[0085] When the substance P applied on the support S is illuminated witha light beam of short wavelength, the back-scatter spot is relativelysmall since there is more absorption.

[0086] Under such circumstances, the image can be enlarged by reducingthe distance between the lens and the sample.

[0087] In a variant, the camera may be provided with a lens providingvariable magnification, enabling the size of the back-scatter spot to bematched to the size of the light-sensitive area.

[0088] The information picked up by the camera is transmitted to acomputer 50 by a cable 51.

[0089] The computer 50 is configured to calculate values that arerepresentative of the capacity of the substance to diffuse and/or absorblight on the basis of the brightness of each pixel in the digital image.The computer 50 can thus be programmed to calculate absorption anddiffusion coefficients.

[0090] Reflectance R(r,φ), i.e. the ratio of the intensity of thereflected light over the intensity of the incident light, can becalculated as a function of cylindrical coordinates starting from thecenter of the spot.

[0091] Integrating R(r,φ) as a function of φ, the function {overscore(R)} (r) is obtained:${\overset{\_}{R}(r)} = {\oint{{R( {r,\phi} )}\frac{d\quad \phi}{2\quad \pi}}}$

[0092]FIG. 4 shows the value of the reflectance {overscore (R)} of thespot integrated as a function of radius r:Rint  (r) = ∫₀^(r)R(ρ)2  π  ρ ⋅ ρ

[0093] A function of the radius is obtained having constants which giveaccess to diffusion and absorption coefficients:${{Rint}\quad (r)} = {a\lbrack {1 - {\exp ( {- \frac{r}{b}} )}} \rbrack}$with  $\quad {a = {a_{1}{\exp \lbrack {- {a_{2}( {1 + \frac{\mu_{s}^{\prime}}{\mu_{a}}} )}^{- 0.5}} \rbrack}}}$$b = {\lbrack {b_{1} + {b_{2}{\ln ( \frac{\mu_{a}}{\mu_{s}^{\prime}} )}}} \rbrack \mu_{s}^{\prime - 1}}$

[0094] and where a₁, a₂, b₁, and b₂ depend only on refractive index n.

[0095] For curve 2 corresponding to the spot of FIG. 2 obtained afterapplying foundation makeup, the following values are obtained:μ_(a)=2.82 cm⁻¹ and μ′_(s)=94.72 cm⁻¹, and for curve 3 corresponding tothe spot of FIG. 3 obtained prior to applying the substance, thefollowing values are obtained: μ_(a)=1.11 cm⁻¹ and μ′_(s)=28.77 cm⁻¹.

[0096] The image of the spot may be subtracted from a black image inorder to cancel out statistically a large fraction of thermal noise.

[0097] It would not go beyond the ambit of the present invention for atleast one polarizer to be placed on the path of the incident light beambetween the source and the cosmetic, and/or on the path of theback-scattered light between the cosmetic and the camera.

[0098] By way of example, FIG. 5 is a normalized image that results fromtaking the difference between images obtained by polarizing the incidentlight beam horizontally and by polarizing the back-scattered light beamfirstly horizontally and/or secondly vertically.

[0099] This operation makes it possible to calculate two additionaloptical parameters, namely the anisotropy coefficient g and the purediffusion coefficient μ_(s).

[0100] For this purpose, three images are acquired: the first withoutpolarization, the second with the incident light and the back-scatteredlight both polarized horizontally, and the third by polarizing theincident light horizontally and the back-scattered light vertically.

[0101] The first image makes it possible to calculate the absorptioncoefficient μ_(a) and the reduced diffusion coefficient μ′_(s), asexplained above, and the other two images makes it possible to calculatethe anisotropy coefficient g and μ_(s) by calculating μ′_(s).

[0102] It would not go beyond the ambit of the present invention to useportable apparatus for acquiring the image of the back-scatter spot.

[0103] By way of example, FIG. 6 shows portable apparatus 60 comprisinga light source and a camera, e.g. of the webcam type, placed in a unit61. The unit 61 has an opening 62 through which the light beam 20 comingfrom the light source exits, and through which the image is acquired bythe camera.

[0104] The data picked up by the camera can be transmitted by means of acomputer 53 over a network 52, in particular the Internet, to aprocessing center 54 connected to a database 55 and configured toprocess the transmitted information and to calculate the reduceddiffusion coefficient and the absorption coefficient.

[0105] The processing center 54 may also be programmed to respond to thetransmitted data and the calculations performed to make a diagnosis andto recommend a suitable cosmetic or care product and to give advice.

[0106] The processing center 54 may also be arranged to enable a varietyof data to be gathered, for example in order to make up a data bankcontaining characteristics representative of the capacities of differentcosmetic substances for diffusing and/or absorbing light.

[0107] The portable apparatus 60 may be present in a beauty parlor or ata point of sale, for example, and may be used for the purpose ofproviding personalized cosmetics and/or care products, in particular asa function of a client's skin type and/or keratinous fibers.

[0108] The substance whose optical characteristics are determined may bea foundation makeup, for example.

[0109] Under such circumstances, the reduced diffusion coefficient andthe absorption coefficient of bare skin can be calculated initially.Thereafter foundation makeup is applied to the skin and the reduceddiffusion coefficient and the absorption coefficient are measured on theskin to which the foundation layer has been applied. By comparing thevalues obtained before and after applying the foundation to the skin, itis possible to evaluate the covering ability of the foundation.

[0110] It is also possible to characterize the match between thefoundation and the skin by proceeding in similar manner. The reduceddiffusion coefficient and the absorption coefficient of the skin beforeand after applying the foundation are calculated, and by comparing thesevalues, information is obtained concerning the capacity of thefoundation for achieving natural makeup.

[0111] The invention also makes it possible to recommend a substancethat matches a particular type of skin or of keratinous fibers.

[0112] To do this, the data bank 55 may contain characteristic valuesfor light diffusion and/or absorption of a range of substances.Thereafter, for a particular individual, it is possible, e.g. by usingthe apparatus 60, to determine values that are characteristic of thelight absorption and/or diffusion of that person's skin and/orkeratinous fibers. These values can be compared with values stored inthe data bank 55, and the processing center 54 can be configured todetermine which substance is most suitable for the individual and tosend this result to the computer 53 over the network 52.

[0113] In another implementation, it is possible initially to apply thesubstance to a support, and then take a first image at a first instantand calculate the reduced diffusion coefficient and/or the absorptioncoefficient for a first time. Thereafter, a second image is taken at asecond instant, for example spaced apart from the first by a timeinterval of about one hour, and the reduced diffusion coefficient andthe absorption coefficient of the substance are calculated a secondtime. By comparing these values during an evaluation step it is possibleto determine how the characteristic light absorption and diffusionvalues vary over time.

[0114] This makes it possible to measure the persistence of a cosmetic,for example.

[0115] Between the first measurement and the second measurement, it isalso possible to take some action, for example seeking to remove thesubstance. It may be constituted, for example, by a sunscreen which isapplied to the skin and said action can be washing. It is thus possibleto determine the ability of the sunscreen to withstand water, forexample.

[0116] The action taken may also be action that serves to modify thephysical and/or chemical properties of the substance and/or of thesupport, in particular the skin. Thus, for example, it is possible tocharacterize the effectiveness of the protection provided by a sunscreenduring exposure to UV radiation, for example by taking account of thereactions of the epidermis that can interact with the substance.

[0117] The substance may also be a depigmenting cream. It is thenpossible to measure the variation in skin pigment over time after thedepigmenting cream has been applied to the skin in order to characterizeits effectiveness.

[0118] The substance may also be an antiwrinkle cream or a moisturizer.

[0119] The invention also makes it possible to determine characteristicvalues for the diffusion and the absorption of a substance applied to aninert support, to the skin, or to keratinous fibers, during thedevelopment of the formulation of the substance in order to determinethe influence of adding certain ingredients so that the formulation canbe modified accordingly, where appropriate.

[0120] Naturally, the invention is not limited to the examples describedabove.

[0121] Thus, it would not go beyond the ambit of the present inventionto calculate values representative of the capacity of a substance todiffuse and/or absorb light other than the reduced diffusion coefficientμ'_(s) and the absorption coefficient μ_(a).

[0122] It is also possible to combine the characteristics of the variousabove-described implementations with one another.

[0123] Throughout the description, the term “comprises a” should beunderstood as being synonymous with “comprises at least one”, unlessspecified to the contrary.

What is claimed is: 1/ A method of determining the capacity fordiffusing and/or absorbing light of a cosmetic or dermatologicalsubstance optionally applied on a support, the method comprising thefollowing steps: a) illuminating the substance and/or a zone of thesupport on which the substance has been applied with an incident lightbeam in such a manner as to form a back-scatter spot; b) acquiring animage of the spot; and c) analyzing the image as a function of variationin brightness between the various points of the spot to determine atleast one item of information representative of the capacity fordiffusing light and/or absorbing it of the substance and/or the supportin the presence of the substance. 2/ A method according to claim 1,wherein a reduced diffusion coefficient μ′_(s) is determined as afunction of the variation in the brightness at various points of thespot. 3/ A method according to claim 2, wherein at least two of saidpoints are situated at different distances from the center of the spot.4/ A method according to claim 1, wherein an absorption coefficientμ_(a) is determined as a function of the variation in the brightness atvarious points of the spot. 5/ A method according to claim 4, wherein atleast two of said points are situated at different distances from thecenter of the spot. 6/ A method according to claim 1, wherein thesupport comprises keratinous fibers and/or cells. 7/ A method accordingto claim 1, wherein the support is inert. 8/ A method according to claim1, wherein the support comprises relief. 9/ A method according to claim1, wherein the surface illuminated by the incident light beam issubstantially plane. 10/ A method according to claim 1, wherein thesubstance is applied on the support over an area in the range 0.5 cm² to5 cm². 11/ A method according to claim 10, wherein the substance isapplied on the support over an area in the range 1 cm² to 2 cm². 12/ Amethod according to claim 1, wherein the incident light beam is a beamof white light. 13/ A method according to claim 1, wherein the incidentlight beam is a beam of monochromatic light. 14/ A method according toclaim 13, wherein the monochromatic light is of red or blue color. 15/ Amethod according to claim 1, wherein the intensity of the incident lightbeam is substantially constant over its entire section. 16/ A methodaccording to claim 15, in which the incident light beam is produced by alight source and wherein a spatial filter is placed between the lightsource and the substance. 17/ A method according to claim 1, wherein theangle of incidence of the incident light beam relative to the normal tothe surface of the substance lies in the range 5° to 25°. 18/ A methodaccording to claim 17, wherein said angle lies in the range 10° to 20°.19/ A method according to claim 1, wherein a mirror is placed on thepath of the incident light beam. 20/ A method according to claim 1,wherein the image is acquired by means of a camera. 21/ A methodaccording to claim 20, wherein said camera is a monochrome CCD camera.22/ A method according to claim 1, wherein the acquired image istransmitted to a processing center in order to be analyzed. 23/ A methodaccording to claim 22, wherein the acquired image is transmitted overthe Internet. 24/ A method according to claim 1, wherein the image isacquired through an optical system of variable magnification. 25/ Amethod according to claim 1, in which the incident light beam isproduced by a light source, wherein a polarizer is placed on the path ofthe incident light beam between the source and the substance. 26/ Amethod according to the preceding claim, wherein the image is acquiredthrough an analyzer. 27/ A method according to claim 1, wherein theimage is acquired in the absence of any interfering light. 28/ A methodaccording to claim 1, wherein the incident light beam is modulated andwherein image acquisition is synchronous. 29/ A method according toclaim 1, wherein the substance is applied on the support with athickness lying in the range 7 μm to 20 μm. 30/ A method according toclaim 29, wherein said thickness is substantially constant. 31/ A methodaccording to claim 1, wherein the substance is of uniform composition.32/ A method according to claim 1, wherein the substance is suitable formodifying at least one physico-chemical property of the support. 33/ Amethod according to claim 1, wherein the substance is selected from thefollowing: foundation makeup, sunscreen, depigmenting cream, antiwrinklecream, moisturizer. 34/ A method of determining a characteristic of acosmetic or dermatological substance, the method comprising the stepsof: determining the capacities of two surfaces to diffuse and/or absorblight, the surfaces being constituted respectively: a) by a substanceapplied to the skin and/or the keratinous fibers; and b) by the skinand/or keratinous fibers in the absence of the substance, by performingfor each surface at least the following steps: i) illuminating a zone ofthe surface in question with an incident light beam in such a manner asto form a back-scatter spot; ii) acquiring an image of the spot; andiii) analyzing the image as a function of variation in brightness atvarious points of the spot in order to determine at least one item ofinformation representative of the capacity of the substance, the skin,and/or the keratinous fibers to diffuse light and/or to absorb it; and:determining at least one characteristic of the substance as a functionof the information obtained for the skin and/or the keratinous fiberswith and without the substance. 35/ A method of determining the way acharacteristic of a cosmetic or dermatological substance varies overtime, the method comprising the following steps: a) applying thesubstance on a support; b) illuminating a zone of the support on whichthe substance has been applied with an incident light beam in such amanner as to form a back-scatter spot; c) acquiring a first image of thespot; d) analyzing the first image as a function of variation inbrightness at various points of the spot to determine at least one itemof information representative of the capacity of the substance todiffuse light and/or to absorb it at a first instant; e) acquiring asecond image of the spot at a second instant; f) analyzing the secondimage as a function of variation in the brightness at various points ofthe spot to determine at least one item of information representative ofthe capacity of the substance to diffuse light and/or to absorb it, atsaid second instant; and g) determining how a characteristic of thesubstance varies over time as a function of the information obtainedduring steps d) and f). 36/ A method according to claim 35, includingthe following step between steps d) and e): exercising an action on thesubstance and/or the support. 37/ A method according to claim 36,wherein said action is at least one of washing action exposure to thewind or to UV radiation making contact with an element on which thesubstance is to be transferred. 38/ A method according to claim 35,wherein the characteristic of the substance which is determined is itscovering ability. 39/ A method according to claim 35, in which thesubstance is a sunscreen, wherein the characteristic which is determinedis the effectiveness of the protection conferred by the substance as afunction of time. 40/ A method according to claim 35, in which thesubstance is depigmenting cream, wherein the characteristic which isdetermined is the effectiveness thereof. 41/ A method of prescribing acosmetic substance, the method comprising the following steps: a)illuminating a zone of the skin with an incident light beam so as toform a back-scatter spot; b) acquiring an image of the spot; c)analyzing the image as a function of variation in brightness at variouspoints of the spot to determine at least one item of informationrepresentative of the capacity of the skin to diffuse light and/or toabsorb it; d) prescribing, at least as a function of said item ofinformation, a cosmetic substance, in particular a substance having thecapacity to diffuse light and/or to absorb light close to the capacitypreviously determined for the skin. 42 Apparatus for determining thecapacity of a cosmetic or dermatological substance to diffuse and/orabsorb light, the apparatus comprising: a source suitable for deliveringa light beam capable of forming a light spot on the substance or on asupport on which the substance has been applied; a camera enabling atleast one image of the spot to be acquired; and an analyzer apparatusfor analyzing said image as a function of variation in brightness atvarious points of the spot to determine at least one item of informationrepresentative of the capacity of the substance to diffuse light and/orto absorb it. 43/ Apparatus according to claim 42, wherein said camerais a monochrome CCD camera. 44/ Apparatus according to claim 42, whereinat least two of said points are situated at different distances from thecenter of the spot. 45/ Apparatus according to claim 42, wherein thelight source comprises a laser. 46/ Apparatus according to claim 42,including a spatial filter through which the light beam passes. 47/Apparatus according to claim 42, including a mirror placed on the pathof the light beam. 48/ Apparatus according to claim 42, wherein theanalysis apparatus is configured to receive the camera image via anetwork. 49/ Apparatus according to claim 48, wherein said network isthe Internet. 50/ A method of preparing a cosmetic or dermatologicalsubstance, the method comprising the following steps: a) illuminating azone of the skin with an incident light beam so as to form aback-scatter spot; b) acquiring an image of the spot; c) analyzing theimage as a function of variation in brightness at various points of thespot to determine at least one item of information representative of thecapacity of the skin to diffuse light and/or to absorb it; and d) atleast as a function of said information, preparing a cosmetic ordermatological substance. 51/ A method according to claim 50, whereinsaid substance has capacity to diffuse light and/or absorb it close tothe capacity previously determined for the skin.